The present invention is directed towards a bus accessing system and method in which a plurality of user terminals communicate with each other via a plurality of bus interface units (BIUs) which are connected to a common transmission bus. More specifically, it relates to a contention bus accessing system and method which are deterministic, i.e. have bounded (maximum) delay on successful information packet transmission.
In known bus communication systems, each of a plurality of BIUs is associated with at least one user terminal, respectively, and periodically places information from its associated user terminal on the transmission bus for transmission to the other BIUs and their associated user terminals. Each BIU formats user information into one or more information packets which generally include: a preamble; a source address (address of BIU and port associated with the user terminal which originated the information packet); a destination address (address of BIU and port associated with the user terminal for which the information is intended); information to be transmitted; and various other information, such as cyclic redundancy check (CRC) bits, etc. Once the BIU has properly formatted the information, it places the information packet on the bus for transmission to the remaining BIUs at instants determined by the particular bus accessing method being used.
One such bus accessing method is known as a time division multiplex system, wherein each user terminal is assigned a predetermined specific time slot or interval during which it can gain access to the transmission bus and place its information packets thereon. Exemplary of such systems is U.S. Pat. No. 3,851,104--Willard et al. While such systems are simple, inasmuch as they avoid the possibility of more than one information packet being placed on the bus during the same interval without the need for any complex intelligence, they are inefficient from a time utilization standpoint since the bus will be idle whenever a BIU has no information to transmit during its assigned interval.
Another bus accessing scheme is a contention system using a listen-while-talk before and during transmission protocol. In this system, the BIU continually monitors the state of the transmission bus and places information packets on the bus whenever the bus is silent, i.e. no other information packets are sensed on the bus. If, during a transmission, the BIU detects other information packets on the bus, it halts its transmission for a predetermined period of time and then attempts to regain access to the bus when the line is silent. Exemplary of such systems are U.S. Pat. Nos. 4,210,780--Hopkins et al. and 4,063,220--Metcalfe et al. One drawback of such a contention system is that it is not prioritized and high priority information may be delayed from access to the bus for substantial periods of time. Another drawback is that the bus can only be loaded to about 40 percent of its maximum information carrying capability without overloading the system.
Still another bus accessing system and method uses token passing. A token is sequentially passed from one BIU to another and only during the time interval a BIU has the token can the BIU place its information packet on the bus. Although this system avoids collisions, the token must be sequentially passed to each BIU. If a BIU has no information packet to transmit this will result in inefficient bus utilization since the bus receives a packet containing only a token and no user information packets during the interval, the BIU having no information packet to transmit controls the token. Exemplary of a token passing system is the token bus standard of the IEEE Project 802 Local Network Standards (Draft C, 1972).
Accordingly, an object of the present invention is to provide a bus accessing system and method wherein fast access to the bus is provided under light bus utilization.
Another object is to provide a bus accessing system and method wherein collisions between information packets on the bus are avoided thereby permitting heavy bus utilization.
Still another object is to eliminate the time necessary to decode a token used in a token passing scheme.
Yet another object is to provide priority for important information.
A further object is to provide equal treatment to all BIU's such that a BIU having completed transmission of an information packet permits other BIU's to attempt to complete transmission of an information packet before again attempting to transmit an information packet.
Still a further object is to provide a system wherein the delay experienced by any BIU to complete a successful transmission of an information packet is bounded, i.e. the delay is limited to a maximum.